Fixes cases where you may be using ActiveModel without ActiveRecord.
Also add a test for loading without ActiveRecord.
The Cucumber feature creates a Gemfile containing ActiveModel and
shoulda-matchers, and then asserts that both can be loaded without any
errors.
The ActiveModel version used will match that of the Rails version
specified in the Appraisal Gemfile; and if no such version can be
determined, the latest will be used.
Closes#480.
